text: For fluid flow through a bed, of approximately spherical particles of diameter  in contact, if the voidage is  and the superficial velocity is , the Reynolds number can be defined as
  or
  or
  The choice of equation depends on the system involved: the first is successful in correlating the data for various types of packed and fluidized beds, the second Reynolds number suits for the liquid-phase data, while the third was found successful in correlating the fluidized bed data, being first introduced for liquid fluidized bed system.
  Laminar conditions apply up to  = 10, fully turbulent from  = 2000.